Perhaps you should try some spelling variations:
nonlinear potential » nonlinear partial, nonlinear material, nonlinear properties
partial theory » particle theory, particles theory, partial tears
spatial theory » special theory, social theory, spatial memory

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (( nonlinear potential theory ) OR ( nonlinear ((partial theory) OR (spatial theory)) )).
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( nonlinear potential theory ) or ( nonlinear ((partial theory) OR (spatial theory)) ))".
• Adding a wildcard symbol may retrieve word variants: (( nonlinear potential theory ) or ( nonlinear ((partial theory) OR (spatial theory)) ))*.